Referring to FIG. 1, a conventional three-phase AC-to-AC power converter 2 is used to drive a three-phase motor 1. The conventional three-phase AC-to-AC power converter 2 includes a rectifying and filtering circuit 21 and an electronic inverter 22. The rectifying and filtering circuit 21 has three input ends (a′, b′, c′) for receiving respectively A-, B- and C-phase AC input power signals from a three-phase commercial power source 10, and rectifies and filters the A-, B- and C-phase AC input power signals to generate a DC voltage signal. The electronic inverter 22 is coupled to the rectifying and filtering circuit 21 for receiving the DC voltage signal therefrom, and is controlled by a controller (not shown in FIG. 1) to generate, based on the DC voltage signal, an R-phase AC output power signal, an S-phase AC output power signal and a T-phase AC output power signal that are supplied respectively to three windings (U′, V′, W′) of the three-phase motor 1 so as to drive the three-phase motor 1.
However, when a misconnection occurs during wire connection, for example, an A-phase terminal of the three-phase commercial power source 10 for outputting the A-phase AC input power signal is unintentionally connected electrically to an R-phase output terminal of the electronic inverter 22, as shown in FIG. 2, one power switch of the electronic inverter 22 may be burned out due to an instantaneous large current directly from the three-phase commercial power source 10, thereby causing malfunctions of or damage to the conventional three-phase AC-to-AC power converter 2.